Mesh Selectivity of Trawl Codend for Japanese Butterfish 〔Psenopsis anomala (Temminck & Schlegel, 1844)〕 off Southwestern Taiwan

Abstract

Japanese butterfish, Psenopsis anomala, is very important to commercial fishing in the northwestern Pacific. This species has traditionally been a steady catch for bottom trawlers in the coastal waters of Taiwan. However, there has been a decrease in catch size, annual landings, and the catch individual/ total hauls (CPUE) of this species. This trend suggests a better management on this species is urgently needed. This study is to compare the selectivity two two-liner codends each equipped with three sizes of net mesh as a means of limiting catches to mature fish only so that immature ones have more time to develop and breed.To perform this study, we used the two codends to perform a total of 24 hauls during nine cruises off the coast of southwestern Taiwan between December 2007 and January 2009. We counted and measured the fish. One two-liner codend, which we labeled Type A, had three mesh sizes 86 mm and 75 mm codends and a 15 mm cover. The other codend, Type B, also had three mesh sizes 75 mm and 60 mm codends and a 15 mm cover. We caught 2551 Psenopsis anomala with fork lengths ranging from 150 and 180 mm. This fish made up the largest part (81.5%) of our catch. Fifty percent of the whole catch were caught with a mesh size of 75 mm. Japanese butterfish with the fork lengths above 140 mm were caught with a mesh size of 60 mm. Based on our logistic regression analysis, the mesh sizes 86 and 75 mm could catch Japanese butterfish with fork lengths of 253.64 and 171.60 mm. The selection spans (SP) of these two mesh sizes were 65.78 mm and 28.87 mm, and their selection factors (SF) 2.94 and 2.28, respectively. Based on the size of Psenopsis anomala once they reach maturity, the Taiwan government recommendation of 20 mm mesh size for bottom trawling should be changed to 70 mm. This would let the immature Psenopsis anomala escape and reduce bycatch, which would contribute to the conservation our resources and improve income over the long term.